diff --git a/ROADMAP.md b/ROADMAP.md
index af891884..cce90a2e 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -205,7 +205,7 @@ Order = user-visible value first; each ships with backend + UI + tests.
 - Own security review on top of upstream fixes: session/token entropy audit, SSRF guard re-test (NAT64 class), rate limiting on auth endpoints
 - Fix upstream bug backlog by impact: ~~#3574 OOM~~ (done — paged tree walk + batched delete), ~~#3118/#3005 WebDAV large-file~~ (done — Content-Range assembly into one session; non-local policies get honest 501; single-PUT giant-file 500s are proxy/client timeouts, not fixable server-side), ~~#3375 SMTP auth discovery~~ (done — `smtp_auth` setting)
 - #3454 (PG FK on upload) is **Pro-only** — `audit_logs` doesn't exist in this codebase. When B.5 adds our own audit log: insert the audit row in the same tx *after* the file row, never before.
-- [ ] #199 (upstream #3584) — markdown editor lag: profile the MDX editor path; likely re-render-per-keystroke, evaluate debounce/virtualization or lighter editor before swapping libraries
+- [x] #199 (upstream #3584) — markdown editor lag: root cause was per-keystroke React re-renders (changedValue state fed back into the editor's initial-markdown prop) re-running every plugin's `update()` hook (RealmWithPlugins has a dep-less effect). `MarkdownEditor` is now `memo`'d with a `useMemo`'d plugins array + stable `translation`; `MarkdownViewer` keeps edits in a ref (read at save) and passes the immutable loaded content — zero re-renders per keystroke
 - [ ] #198 (upstream #3581) — "import files" task shows source storage policy "unknown": check task props → policy name resolution in admin import path (Pro report, likely same code path in CE)
 - [ ] #200 (upstream #3586) — Pro crash on SIGHUP; log shows a clean signal-driven shutdown, no stack trace — watch for a CE repro, likely not actionable yet
 - [x] `desloppify` pass — 73 review items dispositioned (46 fixed, 27 honestly skipped), strict score 77.1 (was 18.9); scorecard lives in README. `security-reviewer` pass done incrementally per batch (OAuth secrets, SSRF, process exec, path safety)
diff --git a/frontend/src/component/Viewers/MarkdownEditor/Editor.tsx b/frontend/src/component/Viewers/MarkdownEditor/Editor.tsx
index 7d53afba..073fc5b4 100644
--- a/frontend/src/component/Viewers/MarkdownEditor/Editor.tsx
+++ b/frontend/src/component/Viewers/MarkdownEditor/Editor.tsx
@@ -42,7 +42,7 @@ import {
 import "@mdxeditor/editor/style.css";
 import { Box } from "@mui/material";
 import i18next from "i18next";
-import { useEffect, useState } from "react";
+import { memo, useCallback, useEffect, useMemo, useState } from "react";
 import { useTranslation } from "react-i18next";
 import "./editor.css";
 
@@ -89,29 +89,12 @@ const MarkdownEditor = (props: MarkdownEditorProps) => {
       setNsLoaded(true);
     });
   }, []);
-  return (
-    <Box
-      sx={{
-        display: "flex",
-        flexDirection: "column",
-        minHeight: props.displayOnly ? "100%" : "calc(100vh - 200px)",
-      }}
-      onKeyDown={(e) => {
-        if ((e.ctrlKey || e.metaKey) && e.key === "s") {
-          e.preventDefault();
-          props.onSaveShortcut?.();
-        }
-      }}
-    >
-      {nsLoaded && (
-        <MDXEditor
-          className={props.darkMode ? "dark-theme dark-editor" : undefined}
-          translation={(key, _defaultValue, interpolations) => {
-            return t("markdown_editor:" + key, interpolations);
-          }}
-          readOnly={props.readOnly}
-          onChange={props.onChange}
-          plugins={[
+
+  // MDXEditor invokes plugin.update() for every plugin on every render;
+  // keeping the array referentially stable lets the realm suppress
+  // downstream updates for unchanged params.
+  const plugins = useMemo(
+    () => [
             diffSourcePlugin({
               diffMarkdown: props.initialValue,
               viewMode: "rich-text",
@@ -244,7 +227,43 @@ const MarkdownEditor = (props: MarkdownEditorProps) => {
               directiveDescriptors: [AdmonitionDirectiveDescriptor],
             }),
             markdownShortcutPlugin(),
-          ]}
+          ],
+    [
+      props.displayOnly,
+      props.initialValue,
+      props.imageUploadHandler,
+      props.imagePreviewHandler,
+      props.imageAutocompleteSuggestions,
+    ],
+  );
+
+  const translation = useCallback(
+    (key: string, _defaultValue: string, interpolations?: Record<string, unknown>) =>
+      t("markdown_editor:" + key, interpolations),
+    [t],
+  );
+
+  return (
+    <Box
+      sx={{
+        display: "flex",
+        flexDirection: "column",
+        minHeight: props.displayOnly ? "100%" : "calc(100vh - 200px)",
+      }}
+      onKeyDown={(e) => {
+        if ((e.ctrlKey || e.metaKey) && e.key === "s") {
+          e.preventDefault();
+          props.onSaveShortcut?.();
+        }
+      }}
+    >
+      {nsLoaded && (
+        <MDXEditor
+          className={props.darkMode ? "dark-theme dark-editor" : undefined}
+          translation={translation}
+          readOnly={props.readOnly}
+          onChange={props.onChange}
+          plugins={plugins}
           contentEditableClassName={props.darkMode ? "markdown-body-dark" : "markdown-body-light"}
           markdown={props.value}
         />
@@ -265,4 +284,7 @@ const MarkdownEditor = (props: MarkdownEditorProps) => {
   );
 };
 
-export default MarkdownEditor;
+// Memoized: the parent re-renders on every keystroke (changedValue state);
+// re-rendering MDXEditor re-runs every plugin's update() hook, which is the
+// dominant source of typing lag on large documents.
+export default memo(MarkdownEditor);
diff --git a/frontend/src/component/Viewers/MarkdownEditor/MarkdownViewer.tsx b/frontend/src/component/Viewers/MarkdownEditor/MarkdownViewer.tsx
index bc55fdb0..bb5941eb 100644
--- a/frontend/src/component/Viewers/MarkdownEditor/MarkdownViewer.tsx
+++ b/frontend/src/component/Viewers/MarkdownEditor/MarkdownViewer.tsx
@@ -1,6 +1,7 @@
 import { LoadingButton } from "@mui/lab";
 import { Box, Button, ButtonGroup, ListItemText, Menu, useTheme } from "@mui/material";
 import React, { lazy, Suspense, useCallback, useEffect, useMemo, useRef, useState } from "react";
+
 import { useTranslation } from "react-i18next";
 import { closeMarkdownViewer } from "../../../redux/globalStateSlice.ts";
 import { useAppDispatch, useAppSelector } from "../../../redux/hooks.ts";
@@ -29,7 +30,10 @@ const MarkdownViewer = () => {
   const supportUpdate = canUpdate(displayOpt);
   const [loading, setLoading] = useState(false);
   const [value, setValue] = useState("");
-  const [changedValue, setChangedValue] = useState("");
+  // Edits are only read at save time; a ref keeps per-keystroke updates from
+  // re-rendering the viewer (and transitively re-running every MDXEditor
+  // plugin update hook).
+  const changedValue = useRef("");
   const [loaded, setLoaded] = useState(false);
   const [saved, setSaved] = useState(true);
   const [anchorEl, setAnchorEl] = useState<null | HTMLElement>(null);
@@ -68,7 +72,7 @@ const MarkdownViewer = () => {
       .then((res) => {
         const content = new TextDecoder().decode(res);
         setValue(content);
-        setChangedValue(content);
+        changedValue.current = content;
         setLoaded(true);
       })
       .catch(() => {
@@ -106,7 +110,7 @@ const MarkdownViewer = () => {
       }
 
       setLoading(true);
-      dispatch(saveMarkdown(changedValue, viewerState.file, viewerState.version, saveAs))
+      dispatch(saveMarkdown(changedValue.current, viewerState.file, viewerState.version, saveAs))
         .then(() => {
           setSaved(true);
         })
@@ -114,11 +118,11 @@ const MarkdownViewer = () => {
           setLoading(false);
         });
     },
-    [changedValue, viewerState],
+    [viewerState],
   );
 
   const onChange = useCallback((v: string) => {
-    setChangedValue(v);
+    changedValue.current = v;
     setSaved(false);
   }, []);
 
@@ -197,11 +201,11 @@ const MarkdownViewer = () => {
       {loaded && (
         <Suspense fallback={<ViewerLoading />}>
           <MarkdownEditor
-            value={changedValue}
+            value={value}
             readOnly={!supportUpdate}
             darkMode={theme.palette.mode === "dark"}
             initialValue={value}
-            onChange={(v) => onChange(v as string)}
+            onChange={onChange}
             onSaveShortcut={onSaveShortcut}
             imagePreviewHandler={imagePreviewHandler}
             imageAutocompleteSuggestions={imageAutocompleteSuggestions}